This invention relates to an air gun used in marine seismic exploration systems as a seismic energy source and more particularly to a method and apparatus for determining when the air gun has been fired.
Marine seismic exploration systems are used to delineate subsurface geologic formations. In marine exploration, one or more air guns generate acoustic energy that travels toward subsurface formations where it is reflected or refracted back to multiple sensors for measurement. The acoustic energy is developed through release of a volume of compressed air or other suitable gas. The release occurs when an air release valve, usually a solenoid valve, is actuated causing a moving part of the air gun, often referred to as the shuttle, to open allowing the compressed gas to rapidly escape. Typically, multiple air guns are deployed in an air gun array and an air gun controller monitors the firing time of each air gun in the array. The air gun controller also controls solenoid valve actuation so that the respective firing times may be synchronized. Poor array synchronization has an adverse effect on acoustic amplitude leading to poor subsurface resolution.
Several methods for monitoring air gun fire times are well known in the art. One method involves the sensing of movement by the air gun shuttle. This method employs a series of magnets that move with the shuttle and a coil of wire located in a stationary position on the air gun. When the shuttle opens, the magnets in the shuttle approach the coil causing the number of magnetic flux lines encircled by the coil to increase. This action induces a voltage in the coil. The voltage is then monitored to determine the precise firing time of the air gun. This time is used to adjust the firing time of each gun so that the peak acoustic output of all air guns in the array are temporally aligned. An advantage to the coil is that it is a passive sensor with low electrical impedance. The coil apparatus and method above is further described in U.S. Pat. No. 4034827, Air Gun Utilizing Magnetized Shuttle, by Leerskov, Jr. et.al., incorporated herein by reference.
The performance of prior art wire coils tends to be degraded when the triggering solenoid valve generates leakage magnetic flux. The solenoid valve generates a magnetic flux with the application of current thereby inducing a voltage in the wire coil. This induced voltage is unwanted and is present even if the air gun did not fire. The amplitude of this signal is also large in comparison to the signal generated by the firing of the gun resulting in a signal-to-noise ratio of less than 10. A higher signal-to-noise ratio is desirable as the signal may be further degraded when transmitted to signal detection electronics external to the air gun.
Another method used to detect the firing time of an air gun is to use an active pressure transducer to monitor internal gun pressure and a passive hydrophone to monitor external pressure. The measured internal gun pressure corresponds to the actual movement of the air gun shuttle and the measured external pressure represents an acoustic pressure rise resulting from firing the gun. This apparatus and method is further described in U.S. Pat. No. 4240518, incorporated herein by reference.
With an active pressure transducer, power must be applied to the transducer for proper operation. This increases the complexity of the electrical interface to the air gun. Also, the active pressure transducer is a high impedance device whose output is significantly degraded with an increase in electrical leakage. Leakage is common in this application, particularly as electrical conductors age and are exposed to harsh environments. The passive hydrophone used to monitor the acoustic pressure wave is usually mounted external to the air gun and is therefore susceptible to mechanical damage under adverse handling conditions.
Along with the deficiencies mentioned above, both the active pressure transducer and the passive hydrophone are costly to replace and fail frequently. The foregoing deficiencies have created a long-felt need for a robust, passive device that is minimally affected by electrical leakage and by magnetic leakage originating from an adjacent solenoid valve.
Accordingly, it is an object of the present invention to provide an apparatus for accurate and economical detection of air gun firing.
A new air gun embodying features of the invention includes a body having a longitudinal axis, a shuttle slidably attached to the body to slide along the body in the direction of the longitudinal axis, a magnet attached to the shuttle, a solenoid valve coupled to the end of the body, and a timing coil assembly attached at the end of the body and sensitive to the motion of the at least one magnet and further adapted to minimize the effect of leakage magnetic flux from the solenoid valve.
In a preferred embodiment, the air gun may include a timing coil assembly with a primary coil sensitive to the magnetic flux of the magnets and the solenoid valve, and a secondary coil sensitive to the magnetic flux of the solenoid valve. In another preferred embodiment, the air gun may include the primary coil substantially concentric to the secondary coil. In another preferred embodiment, the air gun may include the primary coil electrically connected in series with the secondary coil. In another preferred embodiment, the air gun may include the primary coil and the secondary coil with a substantially equal number of wire turns.
Also in accordance with the invention there is provided a marine seismic survey system including at least one air gun as described above, a pressurized pneumatic source in pneumatic communication with the at least one air gun, and an air gun controller coupled to the at least one air gun.
Also in accordance with the invention there is provided a method for detecting the firing time of an air gun including generating a first signal in a primary coil sensitive to the actuation of a solenoid valve and the motion of a magnet on a moving shuttle, generating a second signal in a secondary coil sensitive to the actuation of the solenoid valve, and combining the first signal and the second signal into a third signal so as to cancel the effect of the actuation of the solenoid valve on the third signal.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appending claims.